Blocking oscillator circuits



Sept. 28, 1954 G, D, FORBES 2,690,510

BLOCKING OSCILLATOR CIRCUITS Filed March 29, 1946 Patented Sept. 28, 1954 UNITED STATES PATENT 4OIFLFTIfCE `-BLOCKING :OSCILL'ATOR CIRCUITS .Gordon D. ,-Forbes, Sudbury, `Mass., assigner, by vmesne assignments, to Ithe .United States .of Americaas represented by AtlleSecretary of War .Applicati0n.March29, 1946,1;Serial'No.`657,'93`2 3 Claims.

This invention relates generally .to an oscillator circuit, and, more particularly, toa blocking oscillator circuit.

An object of the present invention is to ,provide a blocking oscillator circuitfadapted Ito-produce in its output .a series of pulses Whichoccur simultaneously with `a controlling input trigger series. t is a Afurther object that the output pulses possess several characteristics such as steep edges, long duration, and high, Yflat top .amplitudes.

Itis another object that the blocking .oscillator function Iwith a series of trigger pulses having "either a Xed or a variable pulse repetitionfrequency (p. r. f.). It is, therefore, another object that the circuit Will recover immediately Ato Y.a responsive condition upon Athe termination of a trigger pulse, so that the oscillator lWill function on a closely following trigger pulse.

Still another object is to provide-ablockingoscillator circuit Which isadapted to-oscillate `freely and produce in its output acontinuous series .of

output pulses'of the type described above.

Other objects, Ifeatures and advantages `of .this

invention will suggest themselves to those skilled inthe art and Willbecome apparent .from .thefollowing description of the invention taken. in .connection with the accompanying drawings `in which:

Fig. 1 is aschematic diagram of a iirst-ernbodiy;

mentof a blocking oscillator `circuit .embodying vthe Yprinciples of this invention;

B is grounded. Cathode I3 is returned through 'resistor I3 to ground potential.

The embodiment shown includes a pulse transformer I5. .The'pulse transformer comprises two windings I6 and I'I, i

which may be of the single Winding, centertapped transformer variety. The windings are `closely .coupled together and lwound on an iron core. The dots indicate `the relative polarity .of

.the vvoltages kinduced in the `transformer wind- `ings .at `the end adjacent to the dot.

positive at the same time.

terminal `2-I. vinput terminal .3| may be applied to grid I2 :the fcircuit i of Fig. l.

fFor :'example, the ends .of .thezwindings havingdots are One end of Winding .ISB iszconnected to'control'grid I2, and theother .end is connected .to `winding I'I, Whichis .the cen-V iter tap'of the windings, andyalsoithrough blocking condenser I9, to cathode I3. The :end of winding I-f'I remote from the center-tap is returned vto Vground potential. The windings are :connected such'thatfor aninorease in-voltage at the cathode, there :will be a -corresponding increase invvoltage at the control grid.

The output is :takenfrom Ycathode I-'3, and is applied through coupling capacitor -Z .tooutput A series of input ytriggerpulses at through couplingcondenser 32.

To facilitate the explanation of the=operation fof 'the'circuit shown in Fig. 1, the voltage-time graphs of Fig. 3 will be referred to. Fig. 3 iniclud'es .two .approximate voltage-.time graphs,

illustrating the voltage -variation at twopointsof The .graphs are plotted ion the l sanietim-e Vscale and reference'axis, with different voltage scales for .each graphbeing used.

Graph A is the approximaterepresentation of the output voltagepulses which appear at cathode I3. Graph B illustrates vthe :approximate voltage :variation :between theicontrol grid and -Lcathode :voltage -V1,indicatedfin Fig. 3, representsthegrid vtocathode cut-01T voltage of the tube. voltage betweengridzandxcathode Which is more .negativethanvoltage V1, tube-Ii! willbeina non- :conductingstate.

`For any -In the'operation of -thel'circuit of F'ig` -1 as 'a free running blocking oscillator, cathode i3 is :originally `abovecut-oif potential, tube I5 is not conducting-and .grid I3 is atground potential due to `the previous operation ofthe circuit. When the `grid to cathode potential -reaches cut-off value, voltage Vi in Fig. 3, tube Iii immediately -starts=to conduct. The instantaneous increase'in fplatecurrent causesvthe voltage of cathode I3 to -go positive. Instantaneous platecu-rrentlflowslby `way of ltwo paths, through resistance zIS, and through winding AI.'I `in `series with condenser I9. The-increasein .potential .at cathode IS-is applied by means ofltransformer I5.to grid I2 as an `increase potential. Asthevgrid is driven more andmorepositive, plate current continues to .in-

crease `until the `tube becomes saturated .in the nonlinearregioh of thetube characteristics. This :action is .cumulative vand rapid. Atsaturation,

.the ,potential oncathode I3 has a .value such as Voltage V4, shown in Fig. 3. While saturated,

the grid-cathode Voltage becomes less and less effective as compared to the plate voltage, which is low, in controlling the current flow through the tube. A transition period occurs, conduction ceases, and the plate voltage is driven quickly back to its original value. This action is again cumulative and rapid.

During the transition period, the grid becomes positive with respect to the cathode, as shown by voltage V2 in Fig. 3, and grid current begins to flow. The potential drop existing across condenser !9 at this time provides a driving potential for the flow of grid current. Grid current does not ilow immediately due to the retarding effect of the windings I and Il through which current must ilow. The transition period is therefore extended as shown by curve A of Fig. 3.

At the end of the transition period, the flow of grid current has charged condenser IQ in the reverse polaritygproducing a negative grid-tocathode potential. The flow of plate current, therefore, begins to decrease, the voltage at cathode I 3, and also at grid I2 decreases. The action is again cumulative and rapid, until the potential at the grid is driven to a low negative value, with respect to the cathode, such as voltage V3 in Fig. 3. Plate current thereby ceases to flow.

The negative charge remaining on condenser I9 then decays exponentially to ground potential, through winding Il and resistor I-S, at a Xed rate determined primarily by the time constant of the network comprising capacitor le and resistor I8. When the potential between the grid and cathode reaches the cutoff value V1, the tube immediately starts to conduct as has been described. A cycle of blocking oscillator action is thereby completed. The blocking oscillator derives its name from the above described chain of events, which is sometimes referred to collectively as a blocking action.

The output is taken from the blocking oscillator from cathode I3, and appears at output terminal ZI. The variation of cathode voltage is shown by graph A of Fig. 3. The output voltage pulse is characterized by having steep leading edges, high amplitudes, and long duration.

As mentioned hereinbefore, condenser I9 discharges through resistance I8 and Winding II. The tube remains cut off for a relatively long period of time, as compared to the period during which the tube conducts a current. The recurrence frequency isy therefore, nearly equal to the inverse of the cut-on" period. A variable resistance, not shown in the figure, may be substituted for resistor i8, and a recurrence frequency control means thereby provided.

The circuit embodying the principles of this invention and having the form shown in the first embodiment is particularly adapted to use a transformer with a single, center-tapped winding. Since the transformer windings are included on the grid-cathode circuit, the windings need not be insulated for high direct-current operating potentials. Such a transformer can be built and adapted to have a minimum leakage inductance and stray capacitances.

Referring to the schematic diagram of Fig. 2, there is shown a second embodiment of the invention including, in the form of a blocking oscillator circuit, a vacuum tube It having an anode EI, control grid I2, and cathode I 3. Anode II is connected through winding 25 of three-winding pulse transformer 2l to the positive terminal of a suitable source of potential, designated B. Control grid i2 is connected to one end of windl ing 26 of pulse transformer 2l. The other end of winding 26 is connected to a second winding 28 of the pulse transformer. rIhe secondary of transformer 2l comprising windings 26 and 28 may be similar in construction to'windings I6 and II, respectively, of transformer I5 shown in Fig. 2. The junction of windings 26 and 28, therefore, form a center tap of the secondary winding of transformer 27, and is connected through capacitor 29'to cathode I3 of tube Ill. Winding 2'5 in the anode circuit of the tube forms the primary winding of transformer 2l. The three-windings are, therefore, mutually inductive. The dots associated with the transformer windings indicate the relative polarity of the voltage induced in the transformer windings as described in connection with the circuit of Fig. 1. The windings of the transformer are connected such that for a decrease in voltage on the plate of the tube, there will be a corresponding increase in voltage at the cathode and also at the control grid. The end of winding 28 remote from the center-tap is returned to ground potential. Cathode I3 is returned through resistor 3l) to ground potential.

Output terminal it is connected to the cathode end of resistor 3B through capacitor 5U. A series of input trigger pulses at input terminal 3l may be applied to grid I2 through coupling condenser 32.

The operation of the circuit in the embodiment shown in Fig. 2 is similar to that described for the embodiment shown in Fig. 1. Instead of a two-winding transformer, the circuit of Fig. 2 includes a three-winding pulse transformer which provides coupling between the three tube electrodes, anode, grid, and cathode. rThe introduction of the additional winding greatly accentuates the blocking action of the blocking oscillator. The circuit is adapted to provide extremely long, square pulses.

Either of the free running blocking oscillators described may be modified to operate in synchronism with a series of input trigger pulses. The positive input trigger pulses applied to input terminal 3l may be applied through coupling condenser 32 to the control grid of the tube. Negative input pulses may be applied to the cathode of the tube. If the input pulses have a fixed frequency, the free-running frequency of the blocking oscillator may be adjusted slightly less than the frequency of the input pulses. When operated in such a manner, the blocking oscillator will produce in its output a series of pulses which occur simultaneously with the controlling input trigger series. With suitable values of the circuit elements, the blocking oscillator will recover immediately after a first blocking action, to become responsive at the termination of a first trigger pulse to a closely following trigger pulse. The frequency of the series of input trigger pulses may vary slightly.

While there has been described hereinabove what is at present considered to be a preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modications may be made therein without departing from the true spirit and scope of the invention.

What is claimed is:

l. A vacuum tube circuit comprising a vacuum tube having at least a control grid, a cathode, and an anode, a multi-winding transformer for coupling said cathode and said control grid, a capacitor, one end of one winding of said transformer being connected to said control grid and the other end of said one Winding being connected through said capacitor to said cathode, one end of a second Winding of said transformer being connected to said other end of said one Winding and the other end of the second Winding being connected to ground potential, one end of a third winding of said transformer being connected to the positive terminal of a source of potential, said anode being connected to the other end of said third winding, all of said windings being inductively coupled to one another, and a resistive impedance connected between said cathode and ground potential.

2. A blocking oscillator, including a vacuum tube having at least a cathode, a control grid, and an anode, and circuits therefor, and multi-winding pulse transformer means for coupling the control grid and the cathode circuits of said tube in such phase that for every change in cathode potential there is a corresponding change of like polarity in control grid potential, said transformer means further coupling the control grid and anode circuits of said tube in such phase that for every change in anode potential there will be a corresponding change of opposite polarity in control grid potential.

3. The oscillator of claim 2, further including means for applying trigger pulses to an electrode of said tube.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,539,402 Nichols May 26, 1925 2,193,850 Andrieu et al Mar. 19, 1940 2,196,825 Giger Apr. 9, 1940 2,212,202 Faudell et al Aug. 20, 1940 2,254,087 Percival Aug. 26, 1941 2,411,573 Holst et al Nov. 26, 1946 FOREIGN PATENTS Number Country Date 40,069 Netherlands Feb. 15, 1937 597,652 Great Britain Apr. 24, 1945 

